Regulation by IFN-beta of Th Cell Differentiation and Function. An effective T helper (Th) cell immune response is dependent upon the activation and differentiation of resting naive CD4+ T cells into memory/effector Th cells displaying appropriate effector functions. Differentiated Th cells can be characterized as functionally distinct subsets according to their pattern of secreted cytokines, which has led to the now established paradigm of inflammatory Th1-like cells (secreting IL-2, IFN-gamma and LT) and anti-inflammatory Th2-like cells (secreting IL-4, IL-5 and IL-10). Aberrant expression of Th1-like cells has been shown to cause inflammatory autoimmune disease in certain animal models, such as experimental allergic encephalomyelitis (EAE) (multiple sclerosis model), experimental colitis (inflammatory bowel disease model) and collagen-induced arthritis (rheumatoid arthritis model). Recently, interferon-beta (IFN-beta), a type I IFN, has been shown to prevent the development of EAE. Furthermore, in patients with multiple sclerosis (MS) IFN-beta provides clinical improvement and is now standard therapy for the disease. The mode of action of IFN-beta in preventing EAE and in treating MS is unknown. We have recently shown, in a series of in vitro studies, that IFN-beta influences human Th cell subset differentiation by 1) preventing the generation of IFN-gamma-secreting Th1-like cells, through the inhibition of CD40-induced IL-12 secretion by dendritic cells, and 2) by inducing the generation of a novel Th cell subset (non-Th1-\Th2-like) that secretes high levels of the anti-inflammatory cytokine IL-10, but does not secrete IFN-gamma, IL-4 or IL-5. We hypothesize that one of the ways in which IFN-beta downregulates inflammatory autoimmune responses is through its effects on Th cell differentiation, specifically, through its ability to prevent the generation of IFN-gamma secreting Th1 cells and to induce a novel IL-10-secreting regulatory Th cell subset. The studies outlined in this research proposal will identify the molecular mechanism(s) by which IFN-beta regulates the secretion of IFN- gamma and IL-10 by human Th cells during their differentiation (Aim #1), and will functionally characterize the human Th cell subset induced by IFN-beta (Aim #2). In addition, murine models will be used to examine the role of both IFN-beta and Th cell subsets induced by IFN-beta in regulating immune responses (Aim #3). The information from these studies will identify mechanism(s) involved in the regulation of Th cell differentiation. The results will provide insight into an often not well appreciated interplay which is integral to the generation of a physiological immune response, that is the interaction between components of the innate immune system (i.e. type 1 IFNs) and the adaptive immune system (i.e. Th cell subsets). Lastly, defining the mechanism(s) by which IFN-beta modulates Th cell differentiation and effector function may not only lead to the development of new, more effective drugs and complementary therapies for the treatment of MS, but could also lead to the use of IFN-beta as treatment of other inflammatory autoimmune diseases.